Osteoporosis affects more than 200 million people worldwide. Therapeutic approaches to osteoporotic bone loss have focused on either anabolic or antiresorptive agents. However, there is a pressing need to develop new agents. NELL-1 is a potent pro-osteogenic protein most studied for its local bone forming effects. Preliminary studies have suggested that NELL-1 also exerts a systemic, protective function against osteoporotic bone loss. Additionally, NELL-1 has recently been identified to have anti-osteoclastic effects, occurring in large part via activation of Wnt/?-catenin signaling. This has ld to our central hypothesis that NELL-1 is a promising systemic therapeutic agent for osteoporosis with ability to (1) effectively reverse osteoporotic bone loss, and (2) regulate bone homeostasis by regulating osteoblast (OB) and osteoclast (OC) differentiation and activity via Wnt/?-catenin signaling. We will test these hypotheses in three specific aims: AIM 1: Optimize systemic recombinant (r)NELL-1 delivery for the reversal of OVX-induced osteoporosis in mice. Our preliminary data show that systemic rNELL-1 reverses OVX-induced bone loss in mice. AIM 1 will test PEGylation of rNELL-1 to optimize systemic delivery in an OVX-induced osteoporotic mouse model. In parallel, we will evaluate the effects of systemic rNELL-1 on stem cell content, OB and OC number and activity, and Wnt/?-catenin signaling. AIM 2: Determine if systemic rNELL-1 can augment anabolic and anti- osteoclastic effects of Wnt/?-catenin signaling. AIM 2 will further explore the extent to which systemic rNELL-1 mediated effects are altered by dysregulated Wnt/?-catenin signaling. In particular, this aim will determine whether rNELL-1 is capable of further enhancing the anabolic effects seen when Wnt signaling is elevated genetically and therapeutically. Two methods of Wnt upregulation will be employed: genetic de- repression of Wnt signaling in all cells via the use of Axin2-/- mice, and pharmaceutical de-repression of Wnt signaling via inhibition of Wnt inhibitor, Dickkopf 1 (DKK1), using neutralizing antibodies. AIM 3: Determine the efficacy of optimized, systemic rNELL-1 for the reversal of OVX-induced osteoporosis in sheep. We have already shown the efficacy of local delivery of rNELL-1 in reversing osteoporotic bone loss in sheep. In AIM 3, we will assess the translational potential of systemic rNELL-1 therapy in an osteoporotic sheep model. Novel rNELL-1 based therapies can improve the current standard of care for the treatment of osteoporosis, and the prevention of osteoporotic fractures that cost $25 billion annually. As well, improved basic biological understanding of rNELL-1 regulation of Wnt signaling may lead to future therapies for other disease entities characterized by deregulated Wnt signaling.